In general, in a liquid chromatograph mass spectrometer, which is a combination of a liquid chromatograph and a mass spectrometer, an atmospheric pressure ionization, such as an electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), is used to generate gas ions from a liquid sample. In the spectrometer of this kind, while the ionization chamber is in an approximately atmospheric pressure, an analysis chamber internally equipped with a detector and a mass analyzer such as a quadrupole mass filter is required to be maintained in a high vacuum state. For this purpose, a differential evacuation system having one or more intermediate vacuum chambers between the analysis chamber and the ionization chamber is used for increasing the vacuum degree in a stepwise manner.
FIG. 6 is a schematic block diagram of the main portion of a conventional LC/MS as disclosed in Patent Document 1 or other documents. This mass spectrometer includes an ionization chamber 11 provided with a nozzle 12 connected, for example, to a column outlet end of a liquid chromatograph (not shown), an analysis chamber 21 internally equipped with a quadrupole mass filter 22 and a detector 23, a first intermediate vacuum chamber 14, and a second intermediate vacuum chamber 18. The first and second intermediate vacuum chambers 14 and 18 are located between the ionization chamber 11 and the analysis chamber 21, and are separated from each other by a partition wall. The ionization chamber 11 and the first intermediate chamber 14 communicate with each other only thorough a desolvation pipe 13 having a small diameter, and the first intermediate vacuum chamber 14 and the second intermediate vacuum chamber 18 communicate with each other only thorough a skimmer 16 having a passage hole (orifice) 17 with an extremely small diameter on top of it.
The internal space of the ionization chamber 11 serving as an ion source is maintained in an approximately atmospheric pressure (about 105 [Pa]) by vaporized molecules of a sample solution continuously supplied thereto from the nozzle 12. The internal space of the first intermediate vacuum chamber 14 as a second stage is evacuated to a low vacuum state of approximately 102 [Pa] by a rotary pump 24. The internal space of the second intermediate vacuum chamber 18 as a third stage is evacuated to a medium vacuum state of about 10−1 to 10−2 [Pa] by a turbo-molecular pump 25, and the internal space of the analysis chamber 21 as the last stage is evacuated to a high vacuum state of about 10−3 to 10−4 [Pa] by another turbo-molecular pump 26. That is, the multistage differential evacuation system in which the vacuum degree of each chamber increases in a stepwise manner from the ionization chamber 11 to the analysis chamber 21 enables the internal space of the analysis chamber 21 as the last stage to be maintained in a high vacuum state.
An operation of this mass spectrometer will be described in outline: A sample solution is sprayed (electrosprayed) from the tip of the nozzle 12 into the ionization chamber 11 while being electrically charged, and molecules of the sample are ionized in the course of vaporization of the solvent in the droplets. The droplets mixed with ions are drawn into the desolvation pipe 13 due to the pressure difference between the ionization chamber 11 and the first intermediate vacuum chamber 14. In the course of passing through the heated desolvation pipe 13, the solvent is further vaporized and the ionization is accelerated. A first lens electrode 15 having a plurality of (four) plate-shaped electrodes arranged in three rows in a sloped manner is located in the first intermediate vacuum chamber 14. This electrode generates an electric field for helping draw the ions through the desolvation pipe 13 and converge the ions around the orifice 17 of the skimmer 16. The ions introduced into the second intermediate vacuum chamber 18 through the orifice 17 are converged by an octapole-type second lens electrode 19 comprising of eight rod electrodes, and sent to the analysis chamber 21. In the analysis chamber 21, only the ions having a specific mass-to-charge ratio (mass/charge) pass thorough the longitudinal space of the quadrupole mass filter 22, and the remaining ions having other mass-to-charge ratios diverge on the way. Then, the ions which have passed through the quadrupole mass filter 22 reach the detector 23, and the detector 23 provides an ionic strength signal corresponding to the amount of the received ions.
In the previously-described mass spectrometer, the first lens electrode 15 and the second lens electrode 19 are collectively called “ion optical system”. Their major function is to converge flying ions with an electric field, and, in some cases, accelerate and send the ions to the subsequent stage. Heretofore, various configurations have been proposed for such lens electrodes. In the example illustrated in FIG. 6, the second lens electrode 19 arranged in the second intermediate vacuum chamber 18 is a multi-rod type as shown in FIG. 7 (while the number of the rods in this example is eight, it may be any even number such as four or six). In this case, a voltage consisting of a radio-frequency AC voltage having an inversed phase superimposed on the same DC voltage is applied to each of the adjacent rod electrodes. Thus, ions introduced along the direction of the ion optical axis C travel while being oscillated at a given frequency by the radio-frequency electric field. This configuration generally has high ability to converge ions; that is, it is capable of sending more ions to the subsequent stage.
With this differential evacuation system, the intermediate vacuum chamber (or chambers) is maintained in a low vacuum (high gas pressure) state, while the mass analysis chamber is maintained in a high vacuum (low gas pressure) state. When ions fly thorough a space of relatively high gas pressure, the kinetic energy of the ions decreases due to the collision with gas molecules existing in the space, resulting in a drop of the flight speed. In particular, when a radio-frequency electric field is applied within the space by the lens electrode as previously described, ions have more chances to collide with the gas molecules because the ions are oscillated by the radio-frequency electric field, and the ions may halt if the length of the radio-frequency electric field is large.
When the flight speed of the ions decreases as previously described, the time for the ions to reach the detector differs even among the ions having the same mass-to-charge ratio, and this causes a decrease in the detection sensitivity and a broadening of a peak. Additionally, when measurements are repeatedly carried out in a scan measurement, SIM (Selective Ion Monitoring) measurement, or other measurements, the ions remaining in the ion optical system may reach the detector in the subsequent measurement and cause a ghost peak, i.e. a peak that appears at a point in time where any peak should not actually appear. The similar problem may possibly occur in the first lens electrode 15; however, this problem is not likely to happen in practice in the first intermediate vacuum chamber 14 because the kinetic energy of the ions is adequately large.
The similar problem is also likely to occur in a tandem mass spectrometer for MS/MS (or MSn) analysis, as well as in the aforementioned type of mass spectrometer in which an atmospheric pressure ionization is used. FIG. 8 is a schematic block diagram of such a mass spectrometer. This mass spectrometer has three stages of quadrupole rod sets 30, 32 and 33 arranged along the ion passageway. The quadrupole rod set 30 in the first stage and the quadrupole rod set 33 in the third stage each function as a quadrupole mass filter for selecting the mass-to-charge ratio of the passing ions as with the quadrupole mass filter 22 in FIG. 6. The quadrupole rod set 32 in the second stage is contained in a collision chamber 31 to which a gas is supplied. When ions are introduced from the left in the figure, only the ions having a specific mass-to-charge ratio are selected by the quadrupole rod set 30 and introduced into the space surrounded by the quadrupole rod set 32 in the second stage. Here, the ions selected in the previous stage collide with gas molecules and are then dissociated. Next, a variety of daughter ions generated according to the dissociation manner are introduced into the quadrupole rod set 33 in the third stage. Finally, the daughter ions having a specific mass-to-charge ratio are selected by the quadrupole rod set 33 in the third stage and reach the detector 34.
In general, only a radio-frequency voltage devoid of a DC voltage is applied to the quadrupole rod set 32 in the second stage so that ions of any mass-to-charge ratio can pass through this stage. However, since the gas pressure of the second stage's internal space is relatively high due to the collision-induced dissociation (CIO) gas which is abundantly supplied, the decrease of the ions' kinetic energy is significant. Hence, if the quadrupole rod set 32 in the second stage is elongated, the ions may stagnate, which causes problems such as the decrease in the detection sensitivity and the appearance of a ghost peak as in the case previously described.
[Patent Document 1] Japanese Patent Publication No. 3379485